When you think about your menstrual cycle, do you think of just your period? For some of us, those few days of bleeding are the only thing we think about, since it’s easy to associate your cycle with the time of the month when you may experience some other physical symptoms like headaches, bloating, and upset stomach.^{16. Committee on Gynecologic Practice, p. 80} But while your period is the part of your menstrual cycle that you actually see, there’s a lot more going on behind the scenes.

The menstrual cycle is made up of many changes that take place inside the body. These changes help prepare a woman's body for pregnancy each month. ^{17. Rebar, p 1495} The progression of the cycle from one to the next is caused by the rise and fall of hormone levels throughout a specific interval, usually about 28 days, or one month. ^{18. Sperroff. p. 195; 19. Fehring p 379B and Table 1} But while the typical menstrual cycle lasts from 21 to 35 days, ^{19. Fehring, p. 379A} it can vary from woman to woman.^{20. Treloar, p. 127 #7; 19. Fehring 382A} Menstrual cycles can also be irregular during puberty, and the years right before menopause.^{10. Berek, p. 159 D; 20. Treloar, p 124 C}

How many days does your cycle last? Chart it with the cycle calendar.

So, what is actually happening inside your body each month? It’s all about hormones. The changes associated with the menstrual cycle are brought on by fluctuations in hormone levels at different times of the month. ^{18. Speroff, p. 195; 21. Silberstein, p. 153} The menstrual cycle can be divided into the following parts: the ovarian cycle and the uterine cycle.^{Berek, p 159}

• The ovarian cycle involves changes in the ovaries, and can be further divided into 3 phases:
  
  
  • The follicular phase is the time from the first day of menstruation until ovulation, when a mature egg is released from the ovary. It’s called the follicular phase because growth or maturation of the egg is taking place inside the follicle, a small sac where the egg matures.^{8, p. 160; 9, p. 718}
    
    
  • Ovulation occurs around day 14 of the cycle, in response to a surge of luteinizing hormone (LH) that occurs just before the egg is released from the ovary.^{10. Berek, p. 160; 17. Rebar p. 1496}
    
    
  • The luteal phase is the time from when the egg is released (ovulation) until the first day of menstruation, when you get your period.^{8, p. 160} It is named after the corpus luteum (Latin for "yellow body"), and is a structure that grows in the ovary where a mature egg was released at ovulation.^{9, p. 422}
    
    
• The uterine cycle involves changes in the uterus. It occurs in tandem with the ovarian cycle, and is divided into two phases ^{8, p. 160}:
  
  
  • The proliferative phase is the time after menstruation and up to ovulation. When menstruation is over, the endometrium (lining of the uterus)^{23. The American Heritage Stedman’s Med. Dic, p. 265} grows and thickens during this phase to prepare for the implantation of an embryo (fertilized egg).^{10. Berek, p 161; 11. Starr, p. 297D}
    
    
  • The secretory phase is the time after ovulation and before the start of a woman's period. Glands within the endometrium secrete proteins in preparation for a fertilized egg to implant. ^{Berek, p. 162} If implantation doesn’t occur, the endometrium begins to break down and the glands stop secreting. The result is shedding of the lining (endometrium), called menstruation. ^{Berek, p. 163}
    

Learn more about the phases of your cycle by getting a closer look at the menstrual cycle.